Changing the Reflectivity of Silica Glass due to Coherent Interference Between Two Laser Beams on Its Surface

We report experimental results of coherent interference between two linearly polarized 532 nm cw-TEMoo laser beams of different brightnesses superimposed on a silica glass surface. The laser beams create an array of aligned dipoles on the surface that act as a diffraction grating. A sinusoidal interference pattern is observed in the parallel component of the reflectance over a 5° range near Brewster angle, of 56.678° for crown glass, with m^th order following the equation: d sin(θ) = m.λ. The experimental angular separation, Δθ = λ/d, between the first three adjacent maxima is 1.571°, for a geometric characteristic d of 0.0194 mm. The interference fringes indicate a change in the optical reflectivity of silica with a reduction at minima and enhancement at maxima. Next, the interaction between laser beams is assisted by a capacitor voltage set across glass for a fine adjustment of the vibrational frequencies of surface dipoles to an isotropic energy environment. We use low voltages for keeping the linear optical response of silica glass to the incident radiation, thus preserving its transparency. The background energy shifts the dipoles’ vibrational frequency by a few eV/h. The brighter coupling laser is oriented at normal incidence on the glass surface and modifies the interaction between the probe laser and the surface dipoles. These dipoles stay aligned along the resultant optical field of the two linearly polarized laser beams, which is slightly adjusted by the static electric field set across the dielectric. Using the Cauchy equation (n² – 1)^-1 = a.λ^-2 + b, with the parameters -7405 and 0.7873 for crown glass irradiated with 532nm, the index of refraction, n (and Brewster angle) shifts from 1.5211 (and 56.678°) at no voltage, to 1.5433 (and 57.059°) at 1.3V, and 1.6053 (and 58.079°) at 3.3V. This reduces Δθ from 1.571° at no voltage, to 1.007° at 1.3V, and 0.649° at 3.3V, according to the equation Δθ ’ = Δθ λ’ / λ, where λ’ is the shifted wavelength due to an assisting voltage. Our results show excellent agreement between theoretical and experimental interference patterns at low voltages. The change in reflectivity of the silica glass surfaces can be used as an optoelectronic switch for retaining the probe's energy on the surface. At fixed incident angle of laser beams, one can find a pair of voltages that switches the surface reflectivity.